Mr. W. Spottiswoode on Stratified Discharges. 535 



space in the line of flow of the current between the particles of the 

 wire produced by the stretching force. 



The conclusions to be drawn from the experiments are : — ■ 



1. That the temporary increase per cent, of resistance of a wire 

 when stretched in the same direction as the line of flow of the 

 current is exactly proportional to the stretching force. 



2. That the increase per cent, of resistance, when a cube of 

 each material is stretched by the same weight, is greater in iron 

 than in steel wire, and greater in brass than in iron ; also that 

 the increase is nearly the same for different specimens of the same 

 material. 



3. That the increase per cent., when a cube of each mate- 

 rial is stretched to the same extent, is much greater in iron and 

 steel than in brass. 



4. That there is a residual increase in each case over and above 

 that which would follow from mere increase of length and diminu- 

 tion of section; that this residual increase is much greater in 

 iron and steel than in brass, and greater in iron than in steel. 



February 15, 1877. — Dr. J. Dalton Hooker, C.B., President, in the 



Chair. 



The following paper was read : — 



" On Stratified Discharges. — III. On a rapid Contact-breaker, and 

 the Phenomena of the Plow." By William Spottiswoode, M.A., F.R.S. 



In a paper published in the Proceedings of the Royal Society, 

 vol. xxiii. p. 455, I have described a form of contact-breaker de- 

 signed for great rapidity and steadiness of action. It consisted 

 of a steel rod which vibrated under the action of an electro- 

 magnet. As regards sharpness of break and steadiness of definition 

 in the striae, this instrument left little or nothing to be desired. 

 But, as explained in the paper above quoted, an alteration in the 

 current not only affected the steadiness directly, but also reacted 

 on the break itself. The effects due to an alteration of the 

 current alone thereby became masked, and the study of the laws 

 relating to such changes was rendered more difficult, or altogether 

 impracticable. In order to obviate this inconvenience I devised 

 another form of contact-breaker, in which the vibrating rod and 

 electromagnet were replaced by an arrangement purely mechanical 

 in its action, and therefore entirely under control. 



This instrument consists essentially of a wheel platinized at the 

 edge, on which a platinum spring rests. In the circumference of 

 the wheel a number (40 in the first instance) of slots were cut, and 

 filled with ebonite plugs so as to interrupt the current. The 

 breadth of the slots was about '04 inch, and that of the teeth 

 about -5 inch. The wheel was connected with suitable driving 

 gear, so as to give from 250 to 2000 currents from the coil in 

 each direction per second. A 4-inch coil was found sufficient to 

 produce the effects ; but the 18-inch coil by Apps, mentioned in 

 former communications, was preferable. With the wheel, as with 

 the electromagnetic break, a very slight strength of current was 



